In a MEMS device, movable MEMS structures are often prone to stiction laterally and/or vertically. In surface micromachined MEMS devices such as accelerometers and gyroscopes, lateral and/or underside stoppers (e.g., bump structures) are often included on the MEMS wafer to prevent over-range movement of the movable MEMS structures. These stoppers are typically configured to have a small contact area with the movable MEMS structures in order to reduce the chance for stiction of the movable MEMS structures to the stoppers. An anti-stiction coating is often also used to enhance anti-stiction performance.
For capped MEMS devices (i.e., where a cap, such as a “dummy” silicon wafer, is bonded to the MEMS device wafer), it also is typical to have vertical stoppers on a cap cavity above the movable MEMS structures, especially when the cavity between the MEMS wafer and the cap is shallow and the movable MEMS structures are in close proximity to structures on the cap.
It is known to bond an ASIC wafer to a MEMS wafer to form an integrated wafer-level chip scale package. In such an integrated wafer-level chip scale package, the ASIC wafer is effectively the cap wafer. Depending on the bond seal material thickness or patterned standoff depth, such devices often have a cavity depth on the order of approximately 2-4 micrometers (abbreviated “um” herein). In such devices, the movable MEMS structures are particularly prone to stiction due to the close proximity of movable MEMS structures to a substantially flat ASIC surface.